The present invention relates to corrosion resistant aluminum alloy sheets for containers and method of producing same. More particularly, the present invention is directed to aluminum alloy sheets useful as metallic can stock, especially as can end stock, for various saline beverages, such as health drinks, tomato juice, etc., food or the like.
Conventionally, mild steel materials, such as tinfree steel sheets or tinplate sheets, have been extensively employed in end parts of cans for the aforesaid saline beverages and other foods. However, it is very difficult to open can ends made of the conventional mild steel sheets because of its high strength and thus there is a risk that the user's hands will be wounded when opening the can.
On the other hand, when aluminum alloy sheets having a easy open property are employed in manufacturing of can ends, the sheets are fabricated from Al-Mg type aluminum alloys, for example, JIS A 5052 and 5082 (throughout this specification, aluminum alloy numbers are represented under Japanese Industrial Standard designations unless otherwise indicated) and a resin coating with a sufficient thickness is applied onto the sheets with a view to protecting the aluminum alloy sheet ends from being corroded by the saline contents. However, it is very difficult for such coating treatment to provide a complete protection coating in industrial production and, thus, it has been for a long time highly desirable to develop corrosion resistant aluminum alloy sheets not suffering corrosion even if the applied protection coatings are incomplete.
As types of corrosions encountered with the conventional cans, there are known a microscopic self-corrosion related to the metallurgical structure of the materials themselves and a macroscopic galvanic corrosion caused from a contact potential between a can end material and can body material. The contact potential arises when different materials are employed in can ends and bodies. Particularly, when a body is formed of mild steel and an end is made of aluminum alloy, such galvanic corrosion phenomenon considerably occurs. Therefore, the galvanic corrosion can not be completely prevented unless the same material is employed in both parts of can bodies and can ends. When tin-free steel, tin plate or the similar mild steels are employed in bodies and ends, the galvanic corrosion is slight, but these mild steels present difficulty as regards to easy opening property of the ends. On the other hand, in the case of using aluminum alloys in can bodies and can ends, for example, JIS A 3004 for bodies and the other different aluminum alloys, such as JIS A 5052 or A 5082, for ends, galvanic corrosion is not negligible.
Further, Al-Mg type aluminum alloys, for example, A 5052, A 5082, A 5182, or the like are employed as can end materials in can manufacturing for low salt content beverages, such as carbonated drinks and beer. In this case, galvanic corrosion is caused by the contact potential between the can end and the mild steel can body with increase in salt content and, thus, the aluminum alloy sheets can not employed as can end stock unless coating having sufficient protection against galvanic corrosion are applied onto them.